1. Field
Embodiments of the present invention relate to lasers and, in particular, to tunable external cavity diode lasers.
2. Discussion of Related Art
An optical telecommunication system transmits information from one place to another by way of an optical carrier whose frequency typically is in the visible or near-infrared region of the electromagnetic spectrum. A carrier with such a high frequency is sometimes referred to as an optical signal, an optical carrier, light beam, or a lightwave signal. The optical telecommunication system includes several optical fibers and each optical fiber includes multiple channels. A channel is a specified frequency band of an electromagnetic signal, and is sometimes referred to as a wavelength. The purpose for using multiple channels in the same optical fiber (called dense wavelength division multiplexing (DWDM)) is to take advantage of the unprecedented capacity (i.e., bandwidth) offered by optical fibers. Essentially, each channel has its own wavelength, and all wavelengths are separated enough to prevent overlap. The International Telecommunications Union (ITU) currently determines the channel separations.
One link of an optical telecommunication system typically has a transmitter, the optical fiber, and a receiver. The transmitter has a laser, which converts an electrical signal into the optical signal and launches it into the optical fiber. The optical fiber transports the optical signal to the receiver. The receiver converts the optical signal back into an electrical signal.
External cavity diode lasers (ECDL) are attracting increasing attention with optical telecommunication system builders as potential widely tunable light sources. A typical ECDL includes a diode laser gain medium with an antireflection-coated facet and a reflective or partially reflective facet, an end mirror, and a wavelength selection element (optical filter). The end mirror and reflective facet form an external laser cavity. The parameters of the gain medium are usually chosen to maximize the output power of the ECDL. The output power of the ECDL is maximized by, among other things, increasing gain medium gain.
One characteristic of increasing gain medium gain to maximize ECDL output power is that the ECDL may become unstable in single mode and begin multimode lasing. This means that the ECDL begins lasing at different frequencies simultaneously or hops between several frequencies. Multimode lasing is acceptable in some applications, such as when the gain medium is used as a pump laser. However, when the gain medium is used in an application such as a tunable ECDL in an optical communication system such instability may be unacceptable. This is because the diode laser must operate within a very tight range of frequencies and cannot hop from frequency to frequency.